1. Field of the Invention
The present invention relates to a high performance current shunt for converting a current to a voltage. In particular, the present invention relates to a high performance current shunt formed from a resistance alloy in sheet form which is sheared, punched, and bent to produce a device which is extremely stable with respect to time and temperature.
2. Description Of The Prior Art
A current shunt for a power supply is typically a four terminal device which converts a current to a voltage. This voltage is used to program the current or to indicate its level. An example of a typical prior art precision current shunt 10 having four terminals is shown in FIG. 1.
As shown in FIG. 1, two terminals are located at each end of the resistive element. One set of terminals, C.sub.1 and C.sub.2, one terminal at each end, are used as current contacts, while a second set of terminals, P.sub.1 and P.sub.2, are used as potential (or voltage sensing) contacts. The potential contacts P.sub.1 and P.sub.2 are typically located inside the current contacts C.sub.1 and C.sub.2 so that they sense a portion of the potential drop in the current path including battery 12 and current contacts C.sub.1 and C.sub.2. A high impedance metering circuit (or voltmeter) 14 is used to monitor this potential drop as shown, and because of its high impedance, no current flows in the potential circuit. The accuracy of the resulting measurement of current is thus largely independent of variations in the quality of the electrical connections to both the current and the potential contacts. This type of connection scheme is sometimes referred to in the prior art as a Kelvin contact. The need for this feature increases as the effective resistance of a shunt decreases since it becomes progressively more difficult to make precisely repeatable connections to the current terminals. Similar, although less stringent, considerations apply to the potential circuit.
As the desired precision of the current shunt increases, the design of the potential connection becomes increasingly critical so that subtler effects on the device's precision must be taken into consideration. For example, prior art four terminal current shunts of the type shown in FIG. 1 achieve modest performance by using screw terminations for the potential connections P.sub.1 and P.sub.2. However, since the potential terminals P.sub.1 and P.sub.2 are located in the current path between the current contacts C.sub.1 and C.sub.2, the screw connections may "creep" slightly with mechanical variations caused by device temperature changes, thus introducing errors by effectively moving the potential contact points P.sub.1 and P.sub.2 in the current path. As still further precision is desired, soldered or welded potential terminals have been proposed in the prior art. Connections of this type avoid the aforementioned problems of mechanical variability; however, since the actual contacts P.sub.1 and P.sub.2 remain in the current path, some portion of the current flows through the solder connection or the weld nugget and through the base of the potential contact lead so as to introduce further errors in the output current measurement.
The materials typically used for the prior art shunt resistance element 10 are generally selected to have very low temperature coefficients of resistance. Solders and the lead materials (which typically are copper) have much higher temperature coefficients of resistance and also have much lower resistivities. Thus, even though the potential contact shunts out a small portion of the shunt resistance element 10 and thus contributes only marginally to the total effective resistance of the device, the potential contact shunts still make a disproportionately large contribution to the overall temperature coefficient of resistance of the finished device since the materials in the region of the potential contact have much higher temperature coefficients of resistance. In addition, because of the dissimilar metals with different thermal EMFs at the potential contacts, temperature differences across the shunt resistance may develop. These effects become increasingly significant as the desired precision of the device is increased. It is desired to develop a current shunt which is specifically designed to minimize these effects.
It is also desired to develop techniques for enabling the resistance value of the shunt resistance element 10 to be very carefully controlled. As known to those skilled in the art, the resistance value of the shunt resistance element 10 varies in accordance with the characteristics of the material used for the shunt resistance element 10 as well as the respective dimensions of the shunt resistance element 10. Accordingly, the resistance value of the shunt resistance element 10 has been controlled in prior art current shunts by carefully selecting the material used for the shunt resistance element 10 and by mechanically trimming (or grinding) the shunt resistance element 10 until it has the desired resistance value. Such an approach is tedious and not easily reproducible. Accordingly, it is desired to extend the thin film or foil resistance element forming techniques used in the manufacture of semiconductor devices to the techniques of manufacturing current shunts from bulk materials so that the resistance of the shunt resistance element 10 may be more accurately controlled.
Accordingly, a need remains in the art for a current shunt which has a resistance value which is extremely stable with respect to time and temperature. It is desired to provide such a current shunt while minimizing manufacturing and development costs.